Various abbreviations that appear below are defined as follows:    CNT carbon nanotube    MEMS micro-electromechanical system    NEMS nano-electromechanical system    PIN personal identification number    RF radio frequency    RSA Rivest, Shamir, and Adelman (a cryptographic algorithm)
Information sent over non-secure connections may allow third parties to intercept, read, copy and use the information for unauthorized purposes. In many cases there is a need to encrypt the information to prevent unauthorized access to sensitive information and to possibly modify the sensitive information. A closely related problem relates to the identification of a sender to control access to some physical or virtual location or information.
As techniques for sender identification and encryption of information are developing, so are also techniques to defeat the identification and encryption. There is a continuous need to develop new and better methods to securely transmit information from one location to another.
Many encryption mechanisms in use (e.g., RSA Secure ID) are based on a secret algorithm and a key that is composed of a code, such as a PIN code, and the time of day and date. These mechanisms together are used to verify that both ends of the link have the same information to form a trusted pair. In case of wireless communications the information encrypted in this way is mapped on a physical carrier and decrypted after demodulation at receiver. However, the secret algorithm may be copied, resulting in its unauthorized use.
Examples of identification in wireless systems may follow the following rules:    a sender transmits frequency masks as an identification key (possibly arranged in a certain prescribed sequence, e.g., cyclically rotated);    a receiver calculates a correlation of a received signal with a locally generated frequency masks (arranged in a certain prescribed sequence); and    an identification/decryption event takes place if a maximum of the correlation function is greater than some certain threshold.
While basically secure, this procedure is also subject to attack by third parties.
In U.S. Pat. No. 5,914,553, “Multistable Tunable Micromechanical Resonators”, Adam et al. describe the use of steady-state chaotic oscillation in a tunable MEM oscillator to provide a mechanism for producing a secure communication system by filtering an information signal through a chaotic MEMS system. An input signal can be encrypted in a first MEMS device, transmitted to a matched MEMS receiver, and decrypted. This is said to be accomplished by fabricating a pair of MEMS oscillators on the same substrate, or wafer, so that they will have very similar, although not necessarily identical, parameters. Even when separated, it is said that they will be sufficiently similar that secure communication between them can be achieved, because of their common origin, by using one oscillator as a filter to produce a noise-like, unintelligible signal, and using the other to recover the information signal through an inverse filter. The result is said to be a symmetrical encryption system whose keys are the chaotic system parameters and the dynamic initial conditions of the system.
While U.S. Pat. No. 5,914,553 suggests the use of tunable MEMS, the secure data communications with a chaotic waveform as described in U.S. Pat. No. 5,914,553 is not practical, since it requires the perfect synchronization between the received chaotic waveform (delayed due to propagation conditions) and a locally generated waveform. In practice any synchronization error due to properties of chaotic signals creates a mismatch that exponentially increases in time between the received waveforms and locally generated waveforms. This mismatch will, after some period of time, destroy a match between the transmitted data and the received data. Furthermore, even with synchronization in place, the secure communications with one chaotic waveform as in U.S. Pat. No. 5,914,553 may be maintained only over a rather limited time period defined by the accuracy of the synchronization.
The use of chaotic dynamics in encryption systems is also described in “Chaotic Circuits and Encryption”, Jun. 16, 2006, where Aimone et al. discuss an ability to synchronize chaotic circuits as being useful to encrypt signals along a communication channel. In this technique a sender encrypts an information signal using a chaotic carrier such that, if the transmission is intercepted, it is chaotic and undecipherable. A receiver removes the chaotic signal from the transmitted signal to obtain the information signal. Ideally, it is said, without the chaotic parameters and equations (e.g., initial conditions), chaos cannot be separated from the signal.
It is known to use arrays of devices to generate random analog vectors with controlled statistics from deterministic chaos. For example, in “VLSI Cellular Array of Coupled Delta-Sigma Modulators for Random Analog Vector Generation”, G. Cauwenberghs (1998 IEEE) reports that cellular arrays of cascaded delta-sigma modulators were used for the purpose of random analog vector generation. In this system the particular form of non-linear coupling between cells is said to not only avoid correlation across cells, but to also produce a truly random sequence in the sense that the outcome of a cell at a given time is statistically independent of its history. The interactions between cells are said to be nearest-neighbor interactions.